The invention relates to the devices for information reading, in particular, from magnetic media of recording and information storage and may be utilized for building magneto-optical heads for information reading.
Known is a head for recording/reproduction of information, containing deposed on an underlay first film of a magnetic material, a film of a non-magnetic material, second film of a magnetic material, and on one side of the head various layers lie in one plane; this side, which becomes the active surface of the head, relative to which moves a magnetic carrier storing information, is covered with a film of a material having zones with various properties, in particular, guard (uprotchnyashtchye) zones (see U.S. Pat. No. 4,897,747, Key No. 360/122, MPK G11B 5/187, 1990). In a particular variant of implementation a thin-film magnetic head of this type contains an underlay, which consists of an aluminum film deposited on a ceramic plate; first film of a magnetic material covered by a film of a non-magnetic material; flat winding embracing the magnetic film; second film of magnetic material isolated from the winding and bound to the first magnetic film. The two magnetic films are in contact with each other in the center of the winding, they have a common zone for passing magnetic flux and a separated by a layer of a non-magnetic material forming the magnetic head gap.
Heads of this type, qualified as inductive heads, are practically difficult to utilize at high and very high densities of information, since the distance between poles of the head has to be finite and its reduction leads to reduced level of useful output signal. In result, with passage to the very high densities the value of useful signal (carrier)/noise ratio sharply drops. Besides, manufacturing of multi-track inductive heads, suitable for simultaneous reading of information from multiple recorded in parallel tracks, appears to be an unusually complex and up to now not finally solved problem.
Known are reproducing heads based on huge magneto-resistive effect (see, for example, Process Complexity of Magneto-resistive Sensors: a Review. R. E. Fontana, Jr. IBM Research Division, The International Magnetics Conference, 1995 DIGESTS of INTERMAG'95, Apr. 18-21, 1995, San Antonio, Tex., USA, pp AA-02). Such a head contains a multi-layer sensitive element, placed between shields and separated from them by two conduction layers. The sensitive element itself is a multi-layer film, consisting of alternating magnetic and conducting layers.
The main disadvantage of the reproducing heads based on the huge magneto-resistive effect is related to the fact, that mass production of such heads required extremely high (nanometric) precision technology, resulting in low yield and high price of such heads. Besides, with the nanometric (and more precisely, sub-nanometric) technology it becomes a very serious problem to produce heads with matching parameters and their utilization in assemblies for multi-track reproduction of information. It is sufficient to point, that in such heads the thickness of the most critical intermediate non-magnetic layer should not vary more than in the range of +-3 Angstroms, with the absolute values of the order of 20 Angstroms. Currently, after more than four years of intensive experiments, it is still impossible to run serial manufacturing of such heads.
There is also a known magneto-optical reproducing head, which may be utilized for a multi-track reproduction, containing a magneto-optical transducer deposited on an underlay, for instance, in form of a thin epitaxially grown layer of ferrite-garnet, which has at least two pole tips made of that magnetic material, separated by a narrow air gap (see U.S. Pat. No. 5,227,938, Key No. 360/114, MPK G11B 5/127, 1993). The transducer, as well as the pole tips, are formed by means of thin-film technology. The material of the magneto-optical transducer exhibits mono-axial (perpendicular to the film plane) anisotropy and appears to form a single domain at least within the area of a single pole tip.
Among the disadvantages of this particular reproduction head one can reckon a low resolution and insufficient sensitivity, which is related to the mono-axial magnetization of the applied magnetic material.
Known is a reproducing head, in its technical essence and that has achieved results closest to the subject of this invention, suitable for a multi-track reproduction, consisting of underlay made of material with a high refraction coefficient, on which deposited are layers of magnetic material separated by a gap: the first-one, active, less than 500 Angstrom thick, made of a soft magnetic material with high saturation magnetization, with magnetization occurring in the plane of the film, and second layer, of greater thickness for shielding and raising the sensitivity (see F. Maurice. Towards the Multitrack Digital Video Tape Recorder. Proceedings of Magneto-Optical Recording International Symposium'91, J. Magn. Soc. Jpn., Vol. 15, Supplement No. S1 (1991), pp 389-394). Magnetic flux, developed by the magnetic carrier of information, is induced in the magnetic structure of the head and is detected using magneto-optical Kerr effect, i.e. based on the rotation of polarization vector of the reading light beam, formed by a laser source. The beam reflected from the active layer of the head is focused on a photo-receiver device, for instance, on a charge coupled device (CCD) which analyses all tracks of recording in parallel. Output represents the time-multiplexed signal.
Low value of the useful signal/noise ratio is to be considered a disadvantage of this known solution, caused by the fact that the rotation angle of the polarized light vector, which is the essential parameter determining the useful signal, is of the order of 0.35 deg. with typically used light wavelengths (650-700 nm) and magnetic fields generated by magnetic carriers of information. Additionally, for the multi-track mode of reproduction with this particular known head significant inter-track disturbances are typical, caused by the fact, that reading surface of the sensitive element of the magneto-optical transducer is either poly-crystalline or amorphous. Another disadvantage of this particular known reproducing head is the poor repeatability of magnetic and magneto-optical properties of the very thin films of the material used for construction the sensitive element, for which usually sendust (Fe, Al, Si alloy) is used. Besides, imperfection of the optical channel, providing detection of the useful signal, does not allow to obtain high sensitivity of said head. Magnetic noise of the area illuminated by the reading light beam, caused by Berkhausen steps effect, leads to an additional reduction of the signal/noise ratio.